Humans have never lived with such high carbon dioxide atmospheric conditions, a new study shows.

These conditions have only become the norm on Earth since 1965.

The study shows that for the entire 2.5 million years of the Pleistocene era, carbon dioxide concentrations averaged 250 parts per million. Today’s levels, by comparison, are more than 410 parts per million. In 1965, Earth’s carbon dioxide atmospheric concentrations exceeded 320 parts per million, a high point never reached in the past 2.5 million years, the study shows.

“According to this research, from the first Homo erectus, which is currently dated to 2.1 to1.8 million years ago, until 1965, we have lived in a low-carbon dioxide environment—concentrations were less than 320 parts per million,” says coauthor Yige Zhang, an assistant professor in the oceanography department in the College of Geosciences at Texas A&M University.

“So this current high-carbon dioxide environment is not only an experiment for the climate and the environment—it’s also an experiment for us, for ourselves.”

Piecing together the history of carbon dioxide

Carbon dioxide is a greenhouse gas that contributes to the warming of Earth’s atmosphere, and is considered a driver of global climate change, Zhang says.

“It’s important to study atmospheric CO2 (carbon dioxide) concentrations in the geological past, because we know that there are already climate consequences and are going to be more climate consequences, and one way to learn about those consequences is to look into Earth’s history,” Zhang says. “Then we can see what kind of CO2 levels did we have, what did the climate look like, and what was the relationship between them.”

The scientists analyzed soil carbonates from the Loess Plateau in central China to quantify ancient atmospheric CO2 levels as far back as 2.5 million years ago. Climate scientists often use ice cores as the “gold standard” in physical climate records, Zhang says, but ice cores only cover the past 800,000 years.

Analyzing Paleogenic carbonates found in the ancient soil, or paleosols, from the Loess Plateau, the scientists reconstructed the Earth’s CO2 levels.

Uncertain future

“The Loess Plateau is an incredible place to look at aeolian, or wind, accumulation of dust and soil,” Zhang says. “The earliest identified dust on that plateau is from 22 million years ago. So, it has extremely long records. The layers of loess and paleosol there contain soil carbonates that record atmospheric carbon dioxide, if we have very careful eyes to look at them.”

“Specifically, carbonates formed during soil formation generally reach carbon isotopic equilibrium with ambient soil CO2, which is a mixture of atmospheric CO2 and CO2 produced by soil respiration,” says Jiawei Da of Nanjing University. “Through the application of a two-component mixing model, we can reconstruct paleo-CO2 levels using carbonates in fossil soils.”

Using those materials and the techniques, the researchers constructed a carbon dioxide history of the Pleistocene.

“Our reconstructions show that for the entire Pleistocene period, carbon dioxide averaged around 250 parts per million, which is the same as the last 800,000 years’ values,” Zhang says.

“Our paleosol-based CO2 estimates are in line with snapshots of early-Pleistocene CO2 retrieved from Antarctic old, blue ice, suggesting that the Earth system has been operating under low CO2 levels throughout the Pleistocene,” says Junfeng Ji of Nanjing University.

We evolved in a low-carbon-dioxide environment, Zhang says, and how humans will evolve and be affected by today’s carbon-dioxide levels is unclear.

The research appears in Nature Communications. Additional coauthors are from Nanjing University in China and the California Institute of Technology.